This invention relates generally to article molding equipment of the reheat and blow type. Particularly, this invention relates to heating apparatus for the reheating of article preforms in the aforesaid molding equipment.
Reheat and blow molding equipment is used to produce finished molded articles from thermoplastic article preforms usually produced by injection molding machinery. The article preforms have a formed portion for accepting an article closure device and a formable portion to be blow molded into the shape of the finished article. The article preforms are loaded in the blow molding machine conveyor, transported through a heating section to raise the preform material temperature to its molecular orientation range, transported therefrom to a molding station at which an expansion fluid is introduced into the preform while it is retained within a mold cavity defining the shape of the finished article, and following expansion, the finished article is ejected for collection. The most time consuming operation of the overall production process is the heating of preforms to the material molecular orientation temperature. Thermoplastics are poor conductors of heat and where convection or radiant heating methods are used, the preform surface temperature elevates above the interior wall section material temperature. The rate of heating is therefore limited by the rate at which heat at the material surface can be transferred to the interior of the wall section. The upper temperature limit for acceptable finished products is the material crystallization temperature and the surface temperature must not exceed this limit. Thus, the rate of heating is further constrained by this maximum surface temperature limit.
In contrast to the aforesaid heating methods, dielectric heating is less dependent on the heat conductivity of the material. Rather, material self-heating is effected by placing the material within an alternating electric field and as the inherently polar molecules of the material move to align their poles with the instantaneous changes of polarity of the field, the kinetic energy is converted to heat within the material. While dielectric heating has been known for some time, adaptation of this technique to high rate production blow molding equipment has heretofore been unknown.
It is therefore an object of the present invention to provide an apparatus for dielectric heating article preforms for a high rate production blow molding machine.
It is a further object of the present invention to provide an apparatus for dielectric heating article preforms having a plurality of electrode pairs.
It is a still further object of the present invention to provide an apparatus for dielectric heating article preforms having contourable electrodes for controlling the temperature profile of the article preform wall section.
Further objects and advantages of the present invention shall become apparent from the following description.